An ancient galaxy appears to be buried inside the Milky Way, new research suggests. 

That’s right: a galaxy inside a galaxy. The lost realm, which the astronomers have dubbed “Loki” after the Norse trickster god, was consumed by our galaxy billions of years ago as it was growing in size, they report in a new study published in the journal the Monthly Notices of the Royal Astronomical Society. 

Loki, if its existence is borne out, would have been a dwarf galaxy. These contain no more than a few billion stars — which is peanuts compared to the hundreds of billions that fully-fledged galaxies like our own contain — and often orbit a larger galaxy. Astronomers are interesting in the formation of dwarf galaxies, which tend to have irregular shapes. Are they smashed together during the gravitational interactions that form a larger one, or brought together by the intervention of invisible dark matter?

Moreover, dwarf galaxies tend to be made of stars that are “metal-poor,” similar to the first stars that first formed in the universe, before later generations fused heavier elements. It was this facet that the astronomers used to divine the presence of the hidden relic.

In the study, they analyzed a group of 20 metal-poor stars in the Milky Way’s galactic plane, the flat disc-shaped region along which most of its stars reside. When they compared their chemical composition to objects in the galaxy’s outskirts, including stars and other dwarf galaxies, they found the chemical traces of numerous cosmic explosions that let off heavy elements, including supernovas and neutron star mergers.

But crucially, they found no traces of a white dwarf explosion. White dwarfs are the dense exposed cores of a medium-mass star like our Sun that shed off all its outer layers. Because it takes billions of years before a white dwarf forms, that suggests that the 20 stars came from an extreme dwarf galaxy that was too short-lived to birth white dwarfs.

Another curious aspect of the suspected dwarf galaxy is that the stars don’t all orbit in the same direction. Eleven are on a prograde orbit, meaning they orbit in the same direction our galaxy rotates, and nine are on a retrograde orbit, which is in the opposite direction. The astronomers’ explanation is an “early accretion”; Loki was merged with the Milky Way when it was still very young and when its own orbits were chaotic, jumbling the stars the dwarf galaxy contained.

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